3.5 was edition I played the most. It was a reason why I quit RPGs for nearly a decade because I hated it so much.
Every time I see another meme about how amazing 3.5 Tarrasque is, I remmember how amogn actual 3.5 players Tarrasque was the biggest joke. It was always brought up as definite proof designers have no idea how to make good monster. It was laughably easy to beat. A wizard could casually solo it, the same abilities people now miss in 3.5 amounted to ribbons. It was a laughingstock, forums had 100+ pages discussions how to fix it and general consensus was it';s beyond saving. It was first proof in 3.5 if you cannot use magic you're only good to roll over and die.
I honestly don't know if everyone claiming 3.5 Tarrasque is such a horrifying monster are trying to rewrite history or unintentionally proving what a broken, unplayable pile of garbage 3.5 was, if it's biggest punching bag is actually dangerous in a different, better designed game.
So it depends on the spell, but I think you are talking about summon nature's ally. That allows you to give instructions to creatures who can understand, and they will fight to the best of their ability, but as a DM I wouldn't interpret the spell as written to include suicide.
But even then, a good DM doesn't put a tarrasque into play and have it sit there and die. Once it realizes it is getting damaged and can't retaliate, it can burrow from we whence it came, etc.
So I think most of the strategies involve weak roleplay from the DM, munchkin builds, liberties with the rules, or both.
Even then, actually killing the tarrasque requires a wish spell, which is not something that a 9th level druid can do.